Techniques for fluid sensing during additive fabrication and related systems and methods

ABSTRACT

According to some aspects, an additive fabrication device is provided comprising a container removably attached to the additive fabrication device, and a detector configured to sense a fluid level of photopolymer resin within the container, wherein said detector does not contact said container.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit as a continuation under 35U.S.C. §120 of U.S. application Ser. No. 15/248,455, filed Aug. 26,2016, which claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 62/211,329, filed Aug. 28, 2015, eachof which is hereby incorporated by reference in its entirety.

BACKGROUND

Additive fabrication, e.g., 3-dimensional (3D) printing, providestechniques for fabricating objects, typically by causing portions of abuilding material to solidify at specific locations. Additivefabrication techniques may include stereolithography, selective or fuseddeposition modeling, direct composite manufacturing, laminated objectmanufacturing, selective phase area deposition, multi-phase jetsolidification, ballistic particle manufacturing, particle deposition,laser sintering or combinations thereof. Many additive fabricationtechniques build parts by forming successive layers, which are typicallycross-sections of the desired object. Typically each layer is formedsuch that it adheres to either a previously formed layer or a substrateupon which the object is built.

In one approach to additive fabrication, known as stereolithography,solid objects are created by successively forming thin layers of acurable polymer resin, typically first onto a build platform and thenone on top of another. Exposure to actinic radiation cures a thin layerof liquid resin, which causes it to harden and adhere to previouslycured layers or to the bottom surface of the build platform.

SUMMARY

The present application relates generally to systems and methods forsensing of a liquid resin within a container of an additive fabrication(e.g., 3-dimensional printing) device.

According to some aspects, an additive fabrication device is providedcomprising a container removably attached to the additive fabricationdevice, and a detector configured to sense a fluid level of photopolymerresin within the container, wherein said detector does not contact saidcontainer.

According to some aspects, an additive fabrication device is providedcomprising a container removably attached to the additive fabricationdevice, three or more capacitive sensors coupled to the additivefabrication device, and an air gap between said container and saidsensors.

The foregoing is a non-limiting summary of the invention, which isdefined by the attached claims.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments will be described with reference to thefollowing figures. It should be appreciated that the figures are notnecessarily drawn to scale. In the drawings, each identical or nearlyidentical component that is illustrated in various figures isrepresented by a like numeral. For purposes of clarity, not everycomponent may be labeled in every drawing.

FIGS. 1A-1B depict an illustrative additive fabrication system,according to some embodiments;

FIG. 2 depicts heating and temperature sensing elements of anillustrative additive fabrication device, according to some embodiments;

FIG. 3 is a flowchart of a method of sensing and heating a liquid resincontainer, according to some embodiments;

FIG. 4 depicts illustrative heating elements of an additive fabricationdevice, according to some embodiments;

FIG. 5 depicts one or more resin level sensors of an illustrativeadditive fabrication device, according to some embodiments;

FIG. 6 is a flowchart of a method of sensing a level of a resincontainer of an additive fabrication device, according to someembodiments;

FIGS. 7A-7B depict illustrative arrangements of capacitive sensingregions of an additive fabrication device, according to someembodiments;

FIGS. 8A-8B depict an illustrative removable container for an additivefabrication device, according to some embodiments; and

FIGS. 9A-9B are schematics showing two different views of anillustrative stereolithographic printer on which aspects of theinvention may be implemented, according to some embodiments.

DETAILED DESCRIPTION

As discussed above, some additive fabrication techniques form solidobjects by solidifying (curing) a liquid, such as a photopolymer resin.It is accordingly important when using such a technique to have asufficient supply of liquid to use for fabrication. While a float orother sensor deployed within a container holding the liquid may providean indication of how much liquid is in the container, such sensorstypically require components to be incorporated into the design of thecontainer. Since the container of an additive fabrication device may bea component that is occasionally replaced, and/or it may be thatmultiple containers are sometimes used (e.g., in a rotation) within asingle device, it can be undesirable to increase the complexity and/orcost of the container. Sensors deployed within a container may alsocause misalignment and/or improper insertion or removal of the containerinto and out of the additive fabrication device. In addition, contactsensors may pose difficulties when used in connection with liquids suchas photopolymer resins. As one example, a photopolymer resin may beincompatible with materials used for contact sensors or immersedcomponents or, by virtue of its chemical properties, may contaminate orcause unwanted material buildup on such sensors due to the propensity ofphotopolymer resin to chemically react to form solid material.

Moreover, additive fabrication techniques that form solid objects from aliquid may produce results that depend upon the temperature of theliquid when it is cured. If an additive fabrication device is calibratedto form solid objects from a liquid under an assumption that the liquidis at a particular temperature, when the liquid is not at thecalibration temperature, objects fabricated using the device may belower in quality than if the liquid were in fact at the calibrationtemperature. In particular, some resin may not completely cure when itstemperature is higher or lower than the calibration temperature, thusleading to possible structural instabilities and/or inaccuracies in thefabricated object.

A temperature sensor may be placed onto a container holding the liquid,but as discussed above, introduction of additional components to thecontainer may be undesirable. Furthermore, a determination of thetemperature would need to be interpreted for the additive fabricationdevice to decide how to adjust fabrication to account for the currenttemperature, which would require extensive calibration of the device atnumerous temperatures.

The inventors have recognized and appreciated techniques for monitoringand controlling conditions of liquid resin within a container of anadditive fabrication device without it being necessary to introduceadditional components to the container itself. In particular, anadditive fabrication device may include sensors configured to determinea liquid resin level within a container and/or a temperature of theliquid resin in the container, without modifications to the container toaccommodate such sensors. The sensors may reside within the additivefabrication device adjacent or close to the container to enablemeasurements of liquid level and liquid temperature. Thus, the additivefabrication device may allow the use of low-cost, replaceable resincontainers whilst automatically maintaining a substantially constantresin temperature and a desired amount of resin in the container duringfabrication.

According to some embodiments, an additive fabrication device mayinclude one or more heaters configured to heat liquid in a container ofthe device to a preselected temperature. This allows the device to becalibrated to operate at the preselected temperature and does notrequire that the device is calibrated to operate at other temperatures.If the preselected temperature is higher than typical ambienttemperatures to which the additive fabrication device may be exposed,this may allow use of the additive fabrication device in any desiredambient environment, yet only requires calibration at a singletemperature (the preselected temperature) to operate under idealconditions. A temperature sensor placed within the additive fabricationdevice may determine the temperature of liquid within a container of thedevice, which may be provided to a heater to indicate whether thetemperature is to be raised or lowered, thereby providing a feedbackloop to establish a stable temperature of the liquid.

According to some embodiments, an additive fabrication device mayinclude one or more sensors configured to sense a level of the liquidresin in a container of the device. The sensors may include one or morecontact and/or non-contact sensors. In some implementations, theadditive fabrication device may include a plurality of capacitivenon-contact sensors configured to sense the liquid level in thecontainer. The additive fabrication device can include an automaticdispensing system to dispense liquid resin into the container based oninformation provided by the sensor(s), thereby providing a feedback loopto establish a desired liquid level in the container.

In the discussion below, various techniques for controlling a liquidresin temperature and level are discussed. It will be appreciated thatthe additive fabrication device performing such techniques can includeone or more processors and/or other suitable controllers to performmonitoring of resin conditions in the device and/or to control elementsof the device to alter the conditions of the resin. For example, thedevice may include a processor configured to receive temperature sensordata indicative of the temperature of resin in a container of the deviceand further configured to send a signal to one or more heaters of thedevice to adjust the temperature of the device based on the receivedsensor data.

To provide an initial overview of the curing process, an illustrativeadditive fabrication system is depicted in FIGS. 1A-1B. Illustrativestereolithographic printer 100 forms an object in a downward facingdirection on a build platform such that layers of the object are formedin contact with a surface of a container in addition to a previouslycured layer or the build platform. In the example of FIGS. 1A-1B,stereolithographic printer 100 comprises build platform 104, container106 and liquid resin 110. A downward facing build platform 104 opposesthe floor of container 106, which contains a photopolymer resin 110.FIG. 1A represents a configuration of stereolithographic printer 100prior to formation of any layers of an object on build platform 104.

As shown in FIG. 1B, an object 112 may be formed layerwise, with theinitial layer attached to the build platform 104. In FIG. 1B, the layersof the part 112 are each formed from the same material but are shown inalternating colors merely to visually distinguish them in this example.The container's base surface may be transparent to actinic radiation,such that radiation can be targeted at portions of the thin layer ofliquid photocurable resin resting on the base surface of the container.Exposure to actinic radiation 115 cures a thin layer of the liquidresin, which causes it to harden. The layer 114 is at least partially incontact with both a previously formed layer and the surface of thecontainer 106 when it is formed. The top side of the cured resin layerbonds to either the bottom surface of the build platform 104 or (in theexample of FIG. 1B) with the previously cured resin layer, in additionto the transparent floor of the container. In order to form additionallayers of the part subsequent to the formation of layer 114, any bondingthat occurs between the transparent floor of the container and the layermust be broken. For example, one or more portions of the surface (or theentire surface) of layer 114 may adhere to the container such that theadhesion must be removed prior to formation of a subsequent layer.

As discussed above, some embodiments of the present inventionadvantageously provide techniques for measuring and/or maintaining thetemperature of liquid resin stored within a container, such as container106. Accordingly, some embodiments of the present invention such as theillustrative embodiment depicted in FIGS. 1A-1B, may include componentsconfigured to raise and/or maintain the temperature of photopolymerresin within container 106 without requiring sensing or heating devicesto be integrated into container 106 or otherwise contact thephotopolymer resin within container 106. Such components mayadvantageously allow for container 106 to be easily removed andreplaced.

FIG. 2 depicts heating and temperature sensing elements of anillustrative additive fabrication device, according to some embodiments.In the example of FIG. 2, stereolithographic printer 200 comprises buildplatform 204, container 206, liquid resin 210, one or more temperaturesensing elements 220, and one or more heating elements 221.

As shown in FIG. 2, a container 206 may be positioned during operationon a support base (not shown) configured with one or more heatingelements 222. In the example of FIG. 2, heating element(s) 221 may beadvantageously located within or beneath a support base immediatelycontacting the container 206, so as not to interfere with the insertionand removal of the container. Any portions of the support base maycontact the container, and may, or may not, include those portions ofthe support base adjacent to heating element(s) 221. A sensor 220 isprovided adjacent to an edge of container 206.

According to some embodiments, regions of the support base locatedbetween heating element(s) 221 and in contact with the container 206 maybe formed of a suitable material that provides structural rigidityand/or has a suitable thermal conductivity that heat generated by theheating element(s) easily conducts to the container, such as, but notlimited to steel, aluminum or other metals. According to someembodiments, heating element(s) 221 may be distributed so as tointroduce heating evenly across the support base. In someimplementations, the one or more heating elements 221 may be located farenough away from sensor(s) 220 so as not to impact the reading of thetemperature of the liquid resin 210 by the sensor(s).

According to some embodiments, heating element(s) 221 may include anyone or more heating elements, such as but not limited to, resistiveheating elements of variable capacity. In some cases, the variablecapacity of such heating element(s) may depend upon thermal propertiesof the additive fabrication system and/or on desired operatingtemperatures. According to some embodiments, a heating element may becontrolled in any suitable way to reach and/or maintain a particulartemperature, including via the use of on-off electrical activation,pulse width modulation activation, and/or other control schemes.

During operation, heating elements 221 may raise the temperature ofregions of the support base located between heating elements and incontact with the container 206 and, via conduction, raise thetemperature of the container. Heat introduced via heating element(s) 221may propagate through the material of the container 206 and, byconduction, into the photopolymer resin within the container 206. Insome embodiments, container 206 may be formed, in whole or in part, froma material relatively resistant to the application of heat, such as, butnot limited to, a polycarbonate plastic such as a UV stabilizedpolycarbonate plastic with a heat distortion temperature per ISO 75/Agreater than 120° C. and/or a Vicat softening temperature, per ISO 306greater than 140° C., such as Lexan Exell D polycarbonate, availablefrom SABIC Innovative Plastics. However, in general suitable materialsinclude any material chemically stable to exposure to desiredphotopolymer materials and mechanically stable in the desired operatingtemperature range.

As shown in the example of FIG. 2, one or more sensors 220 may bemounted adjacent to an edge of container 206. Additionally, oralternatively, one or more sensors may be placed adjacent to the bottomof the container 206.

According to some embodiments, sensor(s) 220 may be located a distance Raway from the edge of container 206 so as to allow for easy removal ofcontainer 206 without the container contacting the sensor(s). Accordingto some embodiments, the distance R may be between 1 mm and 20 mm, suchas between 5 mm and 15 mm, such as between 8 mm and 10 mm.

In some embodiments, sensor(s) 220 may include one or more contactlesstemperature sensors, such as microelectromechanical (MEMs) sensors. Insome embodiments, sensor(s) 220 may include one or more sensorsconfigured to measure the temperature of an object by absorbing infraredenergy from the object including, but not limited to, thermopilesensors. In some implementations, one or more of sensor(s) 220 may be atleast partially covered by a protective shroud, thereby protecting thesensor(s) from exposure to photopolymer resin or collisions. Such ashroud may be chosen from a material such as a thermoplastic (e.g.,HDPE) that does not negatively impact the thermal sensitivity of thesensor(s). In some cases, merely inserting the container into theadditive fabrication device may provide some level of shielding of oneor more of the sensors by placing the container structure between thesensor(s) and the liquid resin.

According to some embodiments, sensor(s) 220 may be positioned such thata measured temperature corresponds to a particular location in thecontainer 206. The inventors have observed that the temperature of anygiven location on the container 206 may be highly correlated with thetemperature of the resin stored within the container 206. Although, ingeneral a relationship between the temperature of each location of thecontainer and the resin temperature may be different for each location.As such, a suitable location of the container can be chosen and thesystem calibrated so that the temperature of the resin may be determinedfrom a measurement of the temperature of the container at the chosenlocation.

Accordingly, in some embodiments, the temperature of the liquid may bedetermined without directly measuring the temperature of the liquid, butrather by measuring the temperature of the liquid container and usingpredetermined calibration data to determine the temperature of theliquid based on the measurement. For instance, calibration data mayinclude temperature correction factors, such as a constant offset orlinear relationship, between temperatures measured by sensor(s) 220 ofthe target location on container 206 and the temperature of the resinstored within container 206. As may be appreciated, it may beadvantageous to delay the taking of measurements using sensor 220 duringperiods of time when the photopolymer resin and container 206 may nothave reached a thermal equilibrium, such as upon the addition ofadditional resin into the container or the beginning of the applicationof heat.

FIG. 3 is a flowchart of a method of sensing and heating a liquid resincontainer, according to some embodiments. Method 300 may be performed bya suitable additive fabrication device, such as device 200 shown in FIG.2.

Method 300 begins in act 301 in which a temperature of a container of anadditive fabrication device is measured. As discussed above, a locationon or within a container may be selected and a relationship between thetemperature of the location and the temperature of liquid resin held inthe container determined. The calibration data representing thisrelationship is stored in data 302 in the example of method 300 and usedin act 304 to calculate a temperature of the resin based on thetemperature measured in act 301.

In act 304, the temperature determined in act 303 is compared with atarget temperature. The inventors have recognized that a targettemperature may be preferably above room temperature, since this allowsthe ambient temperature to cool the liquid resin when its temperaturehas risen above the target temperature. According to some embodiments, atarget temperature may be between 25° C. and 50° C., or between 30° C.and 40° C., such as 35° C.

Irrespective of the target temperature, when it is determined that thetemperature is above the target temperature in act 304, the additivefabrication device may wait for a predetermined length of time beforereturning to act 301 to measure the temperature of the container again.Where the target temperature is above room temperature, this allows theresin to naturally cool via the ambient air during this period. In caseswhere one or more heating element(s) of the additive fabrication devicehave been previously activated, these element(s) may be deactivated inact 306.

When it is determined that the temperature is below the targettemperature in act 304, one or more heating element(s) of the additivefabrication device are activated in act 308 to increase the temperatureof the resin. The length of time during which each heating element(s) isactivated in act 308 may be determined based on the difference betweenthe temperature determined in act 304 and the target temperature. Forexample, the heating element(s) may be activated for a comparativelylonger time when the temperature difference is comparatively larger. Insome cases, the heating element(s) may have a variable heating output,and in such cases both a length of time and an intensity of the heatingoutput may be determined in act 308 and the element(s) activated usingsuch determined parameters. It will be appreciated that on return to act301 from act 308, the heating element(s) may or may not remainactivated, as the element(s) may remain activated through successivepasses through acts 301, 303, 304 and 308 until the target temperaturehas been reached.

FIG. 4 depicts illustrative heating elements of an additive fabricationdevice, according to some embodiments. In the example of FIG. 4, acontainer 402 is positioned above heating elements 401, which are partof an additive fabrication device (not shown). A sensor 404 is also anelement of the additive fabrication device, and is positioned a distance403 away from the edge 407 of the container 402. According to someembodiments, distance 403 may be between 1 mm and 20 mm, such as between5 mm and 15 mm, such as between 8 mm and 10 mm.

As discussed above, some embodiments of the present inventionadvantageously provide techniques for measuring a liquid level of liquidresin stored within a container of an additive fabrication devicewithout requiring sensing or dispensing devices to be integrated intothe container or to otherwise contact the liquid within the container.

FIG. 5 depicts one or more resin level sensors of an illustrativeadditive fabrication device, according to some embodiments. In theexample of FIG. 5, stereolithographic printer 500 comprises buildplatform 504, container 506, liquid resin 510 and one or more resinlevel sensors 530.

It may be generally advantageous to allow for the replenishment ofphotopolymer resin within the container 506 so as to allow for theformation of objects requiring a greater volume of photopolymer resinthan is capable of being stored in the container 506 of a given size. Inaddition, it may be advantageous to limit the volume of the photopolymerresin within the container 506 during operation in order to improveoperation. As one example, lower volumes of photopolymer resin maysubject objects being fabricated to lower overall fluid forces due tofluid flows. As another example, lower volumes of photopolymer resinwithin the working container 506 may reduce the amount of materialwasted during post processing or upon a fabrication failure.

Accordingly, embodiments of the present invention may incorporate areservoir of photopolymer resin 531 and a controller 532 forautomatically controlling introduction of resin from the reservoir intothe container 506. Any suitable delivery system may be utilized in orderto transfer photopolymer resin from a reserve into a container 506. Asone example, controller 532 may include an electro-mechanical valvethat, when activated, induces fluid flow of photopolymer resin into thecontainer 506 via gravitational forces. Additionally, or alternatively,active pumping techniques may be utilized in controller 532.

The inventors have appreciated that, in some cases, introduction ofadditional photopolymer resin into container 506 may interfere withintended operation of the fabrication process by introducing extraneousfluid forces. Accordingly, a way of reducing the disruption of suchresin additions may be desirable.

According to some embodiments, container 506 may be configured toinclude a resin introduction zone 533, located in a region distinct fromthe portions of the container 506 used for exposure of photopolymerresin to actinic radiation. Resin introduction zone 533 mayadvantageously extend laterally from a given portion of the container ata suitable location to interface with controller 532 which transfersphotopolymer resin from reservoir 531. In this way, photopolymer resinmay flow into the container 506 without risk of interference with anobject being formed and without required a substantially largercontainer 506. In addition, resin introduction zone 533 may furtheradvantageously include a sloped feature (e.g., see FIGS. 8A-8B below) soas to direct forces from introduction of resin to locations in whichfabrication is not taking place, to help ensure smooth flow of resininto the container and/or to reduce the likelihood of issues such assplashing or introduction of bubbles into the photopolymer resin.

As discussed above, additive fabrication device 500 includes one or moreresin level sensors 530. Techniques for sensing a level of a liquid maygenerally be divided into contact and non-contact methods. Contactmethods for determining the level of fluid within a container includemechanisms such as floats with positive buoyancy in the intended workingfluid or the completion of electric circuits by conductive fluid. Suchcontact methods, however, may not be well suited for a removablecontainer.

Noncontact liquid sensing methods include techniques such as optical,electrical, or acoustic measurements. Noncontact methods of sensing,however, pose numerous other difficulties, particularly in connectionwith photopolymer materials. One form of noncontact sensing iscapacitive level sensing, which relies upon differences between thedielectric constant of free air, the working fluid, and the material ofthe container in order to affect a measurable capacitance in a way thatcan be mathematically related to the geometry of the fluid within thecontainer. Such sensors may be effective for fluids such as water, whichhas a dielectric constant of approximately 80.4. Typical photopolymerresins, however, have dielectric constants of around 2, which may besimilar to the dielectric constant of one or more plastic materials usedto form the container. In addition, it may be particularly problematicto sense photopolymer resin levels in a removable container, such ascontainer 506 illustrated in FIG. 5, wherein sensing must be performedthrough a container wall located an unknown, and variable, distance fromthe sensor. As one example, “noncontact” capacitive sensing typicallyrequires capacitive sensors to be positioned as close as possible to theliquid to be measured and located a known preset distance from saidliquid, such as mounted onto the side of a container. Such a mountingwould be undesirable for a removable container. These challenges meanthat capacitive sensing has heretofore not been an effective option forsensing photopolymer resin levels in removable container.

Embodiments of the present invention overcome these difficulties, andothers, by taking readings from multiple capacitive sensing regions inorder to determine the level of resin within a container and integratingmultiple capacitive readings in order to compensate for the lowdielectric constant of photopolymer resin, as well as substantialvariability in the position of the container to be measured.

According to some embodiments, sensor(s) 530 may include multiplecapacitive sensors (or, equivalently, multiple capacitive sensingregions, as the number of distinct sensor components is not critical)each connected to a common measuring device (not shown) configured toreceive a reading from each sensor individually. A liquid resin level inthe container 206 may be determined based on the readings received fromthe capacitive sensors and, in some cases, may be determined based alsoon calibration data associated with the capacitive sensors. In somecases, such calibration data may be associated with properties of thecontainer 506, as discussed below.

According to some embodiments, sensor(s) 530 may include multiplecapacitive sensing regions each formed of a conductive material andconnected to a capacitance measuring device (e.g., utilizing a digitaldevice that converts capacitance to some other measuring unit) and toseparate channels of a multi-channel measuring device. According to someembodiments, such a measuring device may be connected to a common groundsource.

In some embodiments, one or more capacitive sensing regions of sensor(s)530 may be surrounded with an active shield. Such active shielding mayadvantageously reduce the influence of noise and sensing artifacts fromfield effects at the edges of each sensing region. In some embodiments,an active shield may be positioned on the side of sensing regions awayfrom the container location, so as to further isolate the capacitivesensing regions from interference. Such a shield may comprise any numberof materials suitable for active shielding, such as copper traces, foil,and/or other conductive material(s).

In some embodiments, one or more capacitive sensing regions of sensor(s)530 may be covered by a protective material to avoid damage, such as avinyl coating. In some embodiments, it may be advantageous for one ormore capacitive sensing regions of sensor(s) 530 to be of approximatelythe same geometry and surface area and in approximately the samelocation and orientation, so as to ensure equal exposure toenvironmental influences. This may allow for easier subtraction ofenvironmental effects since those effects could be assumed to producethe same results in each capacitive sensor when the sensors have thesame size and are located in approximately the same position.

Readings from each individual sensing region of sensor(s) 530 may vary,in part, due to the level of the resin in container 506. However, saidreadings may also vary based on the distance 535 between the sensor(s)530 and the container 506, a distance which in the case of a removablecontainer is variable and typically unknown. In addition, readings mayvary based on a variety of environmental effects causing drift and otherforms of interference. When used in combination, however, the multiplecapacitive sensing regions may be used to determine fluid level height,compensating for environmental interference or drift as well as avariable distance between the sensors and the removable container.

The inventors have recognized that a portion of the capacitive sensingregion signals due to the distance 535 between sensing regions ofsensor(s) 530 and the container 506 may be effectively constant acrosseach capacitive sensing region. Moreover, environmental noise mayinfluence readings from all sensing regions of sensor(s) 530,particularly where said sensing regions are of uniform size, shape, andin substantially the same location. The commonality of these extraneoussignals may make it possible to determine an estimated fluid level basedupon the portion of the signal from the capacitive sensing regions dueto the changing fluid levels. In particular, some embodiments mayutilize the differences and/or gradient, between the capacitive sensingregions of sensor(s) 530 to provide an accurate estimate of the liquidresin level.

According to some embodiments, a level reading from one or morecapacitive sensing regions of sensor(s) 530 may be performed in thefollowing manner. In a first step, a capacitance reading from eachcapacitive sensing region of sensor(s) 530 is taken. Capacitive readingsfrom such sensors may contain noise due to the operation of electricaldevices, electrical instabilities, and/or due to other transientphenomena. Accordingly, it may be advantageous to select a sampling rateand to average values from the sensing regions over a number of samples.In some embodiments, the inventors have experimentally determined thattaking 400-600 samples over a 5-6 second interval provides averagesensor readings with acceptable standard deviations. In someembodiments, such samples are taken separately from each capacitivesensing region of sensor(s) 530, while in some embodiments readings maybe taken of each region concurrently.

In a second step, differences D_(n) between readings for adjacent pairsof sensing regions may be calculated. Once determined, these differencesmay be used to provide accurate estimates of the fluid level of resinwithin the container. In particular, the inventors have determined thatin some implementations the relationship between the values D_(n) isstrongly correlated with the liquid level of the resin within acontainer. As a result, known resin liquid levels may be used in orderto generate a function of D_(n) capable of providing an accurateestimate of resin function.

For example, for three sensing regions in a row there are two adjacentpairs of sensing regions. Differences D₁ and D₂ between readings fromadjacent pairs of regions can be determined, and the estimation functiondetermined:

u×D₁ ²+v×D₂ ²+w×D₁×D₂+x×D₁+y×D₂+z

with experimentally determined parameters u, v, w, x, y, and z ascalibration data. Once calibrated, estimated resin liquid levels may bedetermined by evaluating the polynomial for measured values of D₁ andD₂.

In some embodiments, additional factors impacting the values of D_(n)may be taken into consideration when preparing a mapping between D_(n)and calculated resin liquid levels, including ambient temperature andhumidity.

In some embodiments, calibration data used to interpret the readingsfrom sensor(s) 530 may depend at least in part on properties of thephotopolymer resin 510 within the container 506. This may, for example,be due in part to variable dielectric properties of differentphotopolymer resins.

In some embodiments, calibration data used to interpret the readingsfrom sensor(s) 530 may depend in part on properties of the container506. For instance, the geometry or material used to form the container.In these cases, and others, it may be advantageous to use alternativecalibrations based upon the specific resin and/or container to beutilized. In some embodiments, the user may be required to provide suchidentifications. In other embodiments, however, a container may beidentified based upon a identifier associated with said container. Saididentifier may be capable of being electronically read using anysuitable techniques. Based upon the identifier, the additive fabricationdevice 500 may calculate a liquid resin level based on a particular setof calibration data associated with the identifier. In some cases, suchcalibration data may be baseline calibration data modified usingalteration data retrieved based on the identifier.

In some embodiments, calibration data and/or alteration data may bestored on an electronic media disposed within the container and capableof being electronically read. As will be appreciated, such techniquesmay also be utilized with regards to particular types of resin to beused, particularly in embodiments where resin is stored within areplaceable reservoir which may be so identified for use in determiningappropriate calibration values. Additionally, or alternatively,calibration data and/or alteration data may be stored in a computerrecordable medium of the additive fabrication device.

FIG. 6 is a flowchart of a method of sensing a level of a resincontainer of an additive fabrication device, according to someembodiments. Method 600 may be performed by a suitable additivefabrication device, such as device 500 shown in FIG. 5.

Method 600 begins in act 602 in which a liquid resin level of acontainer of an additive fabrication device is measured. As discussedabove, one or more sensors (e.g., capacitive sensors) may be used tosense the liquid level without contacting the liquid resin. The liquidlevel may be calculated based on calibration data 603, which has beenpreviously determined to allow calculation of the level based onreadings from one or more sensors.

In act 604, the liquid resin level determined in act 602 is comparedwith a target level. When the level is greater than target, no actionmay be taken in the example of FIG. 6 in act 606. Alternatively, whenthe level is lower than a target, in act 608 resin may be automaticallydispensed into the container (e.g., via a reservoir and controller, suchas reservoir 531 and controller 532 shown in FIG. 5).

After act 606 or 608 is performed, the method 600 returns to act 602. Itwill be appreciated that various delays or wait period may be addedbetween any suitable pair of acts in method 600 so that a level issensed at a desired frequency.

In some embodiments, a method similar to method 600 may be performed toautomatically determine whether the level of photopolymer resin within acontainer is at, or approaching, maximum desired levels, so as to avoidoverfilling during a replenishment step.

FIGS. 7A-7B depict illustrative arrangements of capacitive sensingregions of an additive fabrication device, according to someembodiments.

The illustrated sensing regions 702A-C and 752A-C may be arrayed alongthe height of a container (e.g., container 506 shown in FIG. 5)containing resin. To illustrate how the sensing regions are aligned witha container in the example of FIGS. 7A-7B, the z-axis is illustrated.For example, the sensing regions may be aligned vertically along theside of a container, facing the side surface of the container.

As discussed above, one way to calibrate multiple capacitive sensingregions is as a function of the differences between adjacent sensingregions. In the example of FIGS. 7A, difference D₁ may be determinedbased on the difference between the capacitance readings previouslytaken for 702A and 702B. Similarly, difference D₂ may be determinedbased on the difference between the capacitance readings previouslytaken for 702B and 702C.

In the example of FIG. 7B, capacitive sensing region 702B may beextended vertically alongside capacitive sensors 702A, 702B, and 702C.According to some embodiments, capacitive sensors 702A, 702B and 702Cmay have the same shape, the same surface area, or be have same surfacearea and shape.

FIGS. 8A-8B depict an illustrative removable container for an additivefabrication device, according to some embodiments. A suitable containerfor use with the techniques described above is depicted in furtherdetail in FIGS. 8A-8B to illustrate portions of this aspect of thepresent invention.

The illustrated container comprises several features, including anactinically transparent window 801 located in the bottom 811, andsurrounding walls 802, and registration/locking features 803. Theillustrated container also includes a sloped feature 805 which, asdiscussed above, may minimize the effect of resin introduced into thecontainer upon the fabrication process. One example of a suitablecontainer may be found in U.S. patent application Ser. No. 14/734,141,titled “Improved Resin Container for Stereolithography,” filed on Jun.9, 2015, and incorporated herein by reference in its entirety.

FIGS. 9A-9B are schematics showing two different views of anillustrative stereolithographic printer on which aspects of theinvention may be implemented, according to some embodiments.

Illustrative stereolithographic printer 900 comprises a support base901, a display and control panel 908, and a reservoir and dispensingsystem for photopolymer resin 904. The support base 901 may containvarious mechanical, optical, electrical, and electronic components thatmay be operable to fabricate objects using the system. During operation,photopolymer resin may be dispensed from the dispensing system 904 intocontainer 902. Build platform 905 may be positioned along the verticalaxis 903 such that the bottom facing layer of an object beingfabricated, or the bottom facing layer of build platform 905 itself, isa desired distance from the bottom 911 of container 902. The bottom 911of the container 902 may be advantageously transparent to actinicradiation generated by a source located within the support base (notshown) such that liquid photopolymer resin located between the bottom911 of container 902 and the bottom facing portion of build platform 905or an object being fabricated thereon, may be exposed to the radiation.Upon exposure to such actinic radiation, the liquid photopolymer may becured and attached to the bottom facing portion of build platform 905 orto an object being fabricated thereon. (FIGS. 9A-B represent aconfiguration of stereolithographic printer 901 prior to formation ofany layers of an object on build platform 905.) A wiper 906 isadditionally provided, capable of motion over side 907 along thehorizontal axis of motion 910 and which may be removably mounted ontothe support base at 909. The wiper may be coupled to one or moreactuators (e.g., stepper motors, belts attached to motor(s), etc.) thatproduce lateral movement of the wiper across the surface of thecontainer.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those skilled inthe art.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andscope of the invention. Further, though advantages of the presentinvention are indicated, it should be appreciated that not everyembodiment of the technology described herein will include everydescribed advantage. Some embodiments may not implement any featuresdescribed as advantageous herein and in some instances one or more ofthe described features may be implemented to achieve furtherembodiments. Accordingly, the foregoing description and drawings are byway of example only.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Also, the invention may be embodied as a method, of which an example hasbeen provided. The acts performed as part of the method may be orderedin any suitable way. Accordingly, embodiments may be constructed inwhich acts are performed in an order different than illustrated, whichmay include performing some acts simultaneously, even though shown assequential acts in illustrative embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

What is claimed is:
 1. An additive fabrication device comprising: a container removably attached to the additive fabrication device; a dispensing system configured to automatically dispense liquid photopolymer resin into the container; one or more temperature sensors arranged in proximity to the container and configured to generate sensor data indicative of the temperature of the container; and at least one controller configured to: control the dispensing system to dispense liquid photopolymer resin into the container; wait for a predetermined period of time subsequent to said control of the dispensing system; and calculate a temperature of photopolymer resin within the container based on calibration data and based on sensor data produced by the one or more temperature sensors after said wait of a predetermined period of time.
 2. The additive fabrication device of claim 1, wherein the one or more temperature sensors do not touch the container.
 3. The additive fabrication device of claim 1, wherein the container includes a resin introduction zone that is arranged beneath the dispensing system and sloped with respect to an interior surface of the container, such that the dispensing system is arranged to dispense the liquid photopolymer resin onto the resin introduction zone and such that liquid photopolymer resin flows down the sloped resin introduction zone onto the interior surface of the container.
 4. The additive fabrication device of claim 1, further comprising one or more heating elements configured to heat the container.
 5. The additive fabrication device of claim 4, further comprising a support base to which the container is removably attached, wherein the one or more heating elements are arranged within or beneath the support base.
 6. The additive fabrication device of claim 1, wherein the one or more temperature sensors are arranged along a side of the container.
 7. The additive fabrication device of claim 1, wherein the one or more temperature sensors include at least one microelectromechanical (MEMs) sensor.
 8. The additive fabrication device of claim 1, wherein the one or more temperature sensors include at least one thermopile sensor.
 9. An additive fabrication device comprising: a container removably attached to the additive fabrication device; a dispensing system configured to automatically dispense liquid photopolymer resin into the container; one or more temperature sensors arranged in proximity to the container and configured to generate sensor data indicative of the temperature of the container, wherein the one or more temperature sensors do not touch the container; and at least one controller configured to: control the dispensing system to dispense liquid photopolymer resin into the container; and calculate a temperature of photopolymer resin within the container based on calibration data and based on sensor data produced by the one or more temperature sensors.
 10. The additive fabrication device of claim 9, wherein the container includes a resin introduction zone that is arranged beneath the dispensing system and sloped with respect to an interior surface of the container, such that the dispensing system is arranged to dispense the liquid photopolymer resin onto the resin introduction zone and such that liquid photopolymer resin flows down the sloped resin introduction zone onto the interior surface of the container.
 11. The additive fabrication device of claim 9, further comprising one or more heating elements configured to heat the container.
 12. The additive fabrication device of claim 11, further comprising a support base to which the container is removably attached, wherein the one or more heating elements are arranged within or beneath the support base.
 13. The additive fabrication device of claim 9, wherein the one or more temperature sensors are arranged along a side of the container.
 14. The additive fabrication device of claim 9, wherein the one or more temperature sensors include at least one microelectromechanical (MEMs) sensor.
 15. The additive fabrication device of claim 9, wherein the one or more temperature sensors include at least one thermopile sensor. 